The applicant comes with a unique background in medicine and advanced training in geriatric medicine at the UCLA/VA Fellowship Program. He is aiming for an academic career. Hypothyroidism is a disorder of high prevalence in the elderly, thought to be an important cause of neurophysiological deficits and dementia. This study aims to describe the frequency, severity, and specific characteristics of the cognitive and neurophysiological abnormalities caused by hypothyroidism, as well as to examine its reversibility by thyroid hormone. The specific aims of this study are: 1) To determine the specific changes in cognitive function that are associated with hypothyroidism of varying severity and duration in the elderly. 2) To determine the characteristics of auditory evoked potentials (EP), an indicator of neurophysiologic function, in the same groups of hypothyroid elderly. 3) To determine whether changes which may be found in cognitive and neurophysiologic function are reversible when the euthyroid state has been attained. The study patient population will consist of the following groups. 1) Control group: (65 or older) with normal thyroid function and no apparent cognitive dysfunction. 2) Hypothyrodism, chronic. a. Compensated hypothyroidism: geriatric patients, diagnosed by routine screening, who are found to have an elevated TSH of 10mU/L or greater with normal FT4I (5-12). b. Mild hypothyroidism patients with moderately low FT4I (3.5-4.5) and TSH of 10mU/L or greater. c. Clinically evident hypothyroidism, FT4I less than 3.5 and TSH less than 20 mU/L. 3) Short term hypothyroidism: patients of all ages with carcinoma of the thyroid who are taken off thyroid medications for a short period to prepare for I131 scanning in search for residual tumors. Mental State Exam and evoked potentials P300 and P100 of each individual from groups 2 and 3 will be carried out upon diagnosis of hypothyroidism and retested when fully replaced with thyroid hormone. Individuals in group 3 will be tested while on a full replacement hormone therapy and retested when taken off thyroid hormone for scanning. Group 1 will be tested twice, 8 weeks apart, for test-retest score difference measurement. Investigation into the relationship between mental state test scores and evoked potentials will enhance our understanding of the neurophysiology and function of the aging brain in general and hypothyroidism in particular. This study links excellent people from three disciplines interested in the study and in educating the candidate. The candidate can learn research techniques in endocrinology, neurophysiology and geropsychiatric aspects of aging.